When tikzposter package is installed only some of its components are extracted: tikzposter.cls, readme and tikzposter.pdf. All acompaining theme files (which are included in tikzposter.dtx file and extracted with
latex tikzposter.ins) are not installed. I think they should be in folder where .cls is, but it's just my opinion. There are also two more files: tikzposter-example.tex and tikzposter-template.tex that I don't know where to place.